Inevitable issues in identifying mpox respiratory transmission – Authors' reply
Authors: Amy Beeson, Ashley Styczynski, Mary G Reynolds, Andrea M McCollum, Sarah Anne J Guagliardo
Journal: The Lancet Microbe (2023)

We thank Byung Uk Lee for pointing out that respiratory transmission of mpox (formerly known as monkeypox) cannot be excluded during close contact between people, and that viral concentration and duration of exposure affect transmission risk. We agree on both points but maintain our overall conclusion that respiratory transmission was probably not a substantial contributor to person-to-person spread in the 2022–23 global outbreak. In our Personal View,1Beeson A Styczynski A Hutson CL et al.Mpox respiratory transmission: the state of the evidence.Lancet Microbe. 2023; 4: e277-e283Summary Full Text Full Text PDF Scopus (1) Google Scholar we acknowledge that: “The nature of close contact… might entail various combinations of acquisition routes, with confounding exposures”. Although all exposures are confounded by shared air, it is striking that, to date, there have been no reports of respiratory transmission events in which physical contact and fomite transmission could be excluded. A recent systematic review of 3924 cases found no cases in which inhalation could be isolated as the acquisition route.2Kuehn R Fox T Guyatt G Lutje V Gould S Infection prevention and control measures to reduce the transmission of mpox: a systematic review.MedRxiv. 2023; (published online Feb 15.) (preprint).https://doi.org/10.1101/2023.02.13.23285871Google Scholar The clinical presentation of most cases with primary lesions localised to the genitalia instead supports direct inoculation during sexual contact.3Thornhill JP Barkati S Walmsley S et al.Monkeypox virus infection in humans across 16 countries—April–June 2022.N Engl J Med. 2022; 387: 679-691Crossref PubMed Scopus (629) Google Scholar Furthermore, the importance of viral concentration and duration of exposure relative to other factors is unclear. The infectious dose and aerosol viability of monkeypox virus (MPXV) are unknown and measurements of quantum generation, which would aid in predicting respiratory transmission risk, have not been performed.4Buonanno G Morawska L Stabile L Quantitative assessment of the risk of airborne transmission of SARS-CoV-2 infection: Prospective and retrospective applications.Environ Int. 2020; 145106112Crossref PubMed Scopus (210) Google Scholar Given the low R0 estimates of mpox compared with smallpox, we would argue that the infectious dose of MPXV cannot be extrapolated from variola virus and is likely to be greater than a single virion. Of note, despite detecting a high concentration of viral DNA in some air samples, Hernaez and colleagues5Hernaez B Muñoz-Gómez A Sanchiz A et al.Monitoring monkeypox virus in saliva and air samples in Spain: a cross-sectional study.Lancet Microbe. 2023; 4: e21-e28Summary Full Text Full Text PDF PubMed Scopus (12) Google Scholar were unable to recover infectious virus from 42 air samples. We maintain our assessment that respiratory transmission is not currently a major pathway of human-to-human MPXV transmission. In addition to aerosol dynamics, observed epidemiological and clinical patterns and other factors are worthy of consideration when assessing transmission routes and making public health recommendations. We declare no competing interests. The findings and conclusions in this Correspondence are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention. Mpox respiratory transmission: the state of the evidenceThe relative contribution of the respiratory route to transmission of mpox (formerly known as monkeypox) is unclear. We review the evidence for respiratory transmission of monkeypox virus (MPXV), examining key works from animal models, human outbreaks and case reports, and environmental studies. Laboratory experiments have initiated MPXV infection in animals via respiratory routes. Some animal-to-animal respiratory transmission has been shown in controlled studies, and environmental sampling studies have detected airborne MPXV. Full-Text PDF Open AccessInevitable issues in identifying mpox respiratory transmissionIn their Personal View of the respiratory transmission of mpox (formerly known as monkeypox),1 Amy Beeson and colleagues overlooked two important physical aspects and drew challenging conclusions. Full-Text PDF Open Access